Method and apparatus for improving combustion in recovery boilers

ABSTRACT

A combustion air system for a recovery boiler is described in which multiple levels of secondary and tertiary combustion air ports each have an even number of ports, with the ports on opposing walls interlaced. The air system lends itself equally well to front/rear wall or sidewall applications and is especially beneficial for rectangular boilers. The air system features large and small-scale horizontal circulation zones superimposed on each other and the ability to adjust the angle of the air jets. Additional features include port dampers for the starting burners and system control based on kinetic energy.

BACKGROUND OF THE INVENTION

Recovery boilers are well known in the pulp and paper industry as ameans to recover spent cooking chemicals and the associated heatingvalue to produce steam for process use or power generation. The spentcooking chemicals are recycled after being used to dissolve wood chipsto liberate fibers for papermaking. The fibers are separated from thechemical bath, which contains a high concentration of organic materialthat can be burned in the recovery boiler. The “spent” cooking chemicalsare recovered from the chemical bath through the combustion process. Inorder to recover the spent chemicals and burn the organic matter, muchof the water is evaporated from the chemical stream, with the resultantforming concentrated “black liquor” with upwards of 75% solids content(organic and inorganic materials). This black liquor is sprayed into theboiler in an atomizing fashion forming droplets that dry and go throughseveral processes, and expel flammable gasses and char material. Toactivate some of the cooking chemicals, they are chemically reduced,which requires high heat. Since the total cooking chemicals areinorganic, almost all fall to the floor of the boiler in the form ofmolten smelt that flows out of the bottom of the recovery boiler to bedissolved, processed and reused. The finer points of recovery boilerdesign and operation are described in detail in many patents, several ofwhich are cited below. The black liquor is sprayed into the furnace byone or more injection nozzles at an elevation of from 4 to 10 meters ormore. Combustion air enters the boiler at several levels via portopenings arranged around the perimeter of the boiler, some levels aboveand some below the liquor spray. The interaction of the combustion airand flammable materials inside the boiler is crucial for the boiler toperform well. In particular, improving the mixing of the air and fuelimproves the combustion and many dependent process variables. Being offinite size and heavily loaded, many recovery boilers are at the limitof their ability to process black liquor while using outdated combustionair systems. The combustion air system consists of all of the designparameters and components required to introduce combustion air into theboiler. This includes fans, air heaters, ducting, dampers, portcleaners, instrumentation, controls, actuators, and the size andarrangement of the port openings themselves. The port openings are theopenings in the walls of the furnace through which the combustion airenters. The present invention is focused on an improved arrangement ofcombustion air ports for a recovery boiler.

Typical recovery boilers consist of a floor and walls constructed fromheavy steel tubing, welded together forming walls with the tubes runningvertically. The walls and floor form a large box that contains thecombustion. The tubes are filled with water that circulates through thefloor and walls and absorbs heat from the combustion in the boiler. Thewater eventually flows upward to the convective heat transfer sectionslocated at the top of the boiler. These include the screen tubes,superheater and generating bank. Combustion air is typically injectedinto the furnace at a variety of levels, with the primary and secondarylevels located below the liquor spray, and the tertiary and higherlevels located above the liquor spray. Some boilers have combustion airintroduced at or very close to the liquor spray level. There may be fromone to over ten different levels of combustion air. Many arrangements ofcombustion air systems are described in the literature and patents, someof the more pertinent examples being U.S. Pat. No. 5,121,700 (Blackwellet al.), U.S. Pat. No. 5,305,698 (Blackwell, et al.), U.S. Pat. No.5,724,895 (Uppstu), and U.S. Pat. No. 6,302,039 (McCallum et al.). Manyof these concepts have been tried on operating boilers and have yieldedvarying degrees of success. Until recently it has not been possible totest a combustion air system concept thoroughly. Prior to the advent ofadvanced computational fluid dynamic (CFD) modeling, engineers had torely on experience, similitude, and mathematical models to predict theperformance of a combustion air system design. CFD modeling techniquesand software, combined with high performance computers, now permits theaccurate, comprehensive, and economical testing of combustion airsystems, and the comparison of many different designs. While some CFDmodeling may have been used in the development of the above patents, itwas not of the sophistication that is currently available. Extensive“estate of the art” CFD modeling was used to develop the presentinvention and to test various combustion air systems including many ofthose cited in the above references. The Combustion air system describedin this application has been shown to outperform the older combustionair system designs in a variety of ways.

The invention described here is mainly concerned with the arrangement ofthe combustion air port openings through the boiler walls. Thearrangements of the fans, heaters, ducting, etc., are typically employedaccording to common engineering practice, with the exceptions detailedbelow. It has been revealed by experience and CFD modeling that the airjets emitted from the combustion air ports must be at least partiallyinterlaced in order to be effective at mixing the fuel and air whilelimiting the carryover associated with high vertical gas velocities inthe boiler. Carryover is the particulate matter that is a by-product (orportion of) the black liquor sprayed into the boiler that is entrainedin the vertical gas flow. The combustion air mainly flows upward in theboiler carrying particles to the convective heat transfer surfaces whereit can eventually plug the entire boiler. High vertical velocities andpoor mixing also carry high temperatures into the upper boiler becausecombustion is delayed, and transport times are faster. The combinationof high carryover and high temperatures causes rapid fouling in theupper furnace.

Many older combustion air systems employ air ports arranged in severallevels. A “level” consists of all those combustion air ports arranged atabout the same elevation on the boiler and excludes burner ports, cameraports, and etc. The primary level typically consists of one or twohorizontal rows of air ports on all four sides of the boiler. Theprimary level is the lowest level in the boiler and may supply up to 50%or more of the total required combustion air. Above the primary levelbut below the liquor spray is the secondary level or levels. Commonpractice is to have a single secondary level but zero to four or morelevels have been tried. The secondary level may supply up to 50% of thetotal combustion air. Above the liquor spray is the tertiary level. Thetertiary most typically consists of a single level but may have 6 ormore levels. The tertiary may supply up to 50% of the total combustionair, but 20% is more typical. Some boilers are fitted with a quaternarylevel above the tertiary, but the delineation is often merely semantics.For the sake of this discussion, all levels above the liquor spray willbe referred to as tertiary air, unless that level is fed with somethingother than combustion air (e.g. re-circulated flue gas or dilutenon-condensable gasses).

An interesting development in recovery boiler air systems is describedin the Uppstu patent, U.S. Pat. No. 5,724,895. This patent details a“vertical” air system with many secondary and tertiary levels. This“vertical” system has many air levels, but practice has shown that it isvirtually impossible to use all these levels and openings because, to dosuch would mean that the airflow mass at each opening would be toolittle to have any influence on the mixing of air and fuel. The energysupplied with the combustion air system supplies the mixing energy forcombustion, and if the airflow from an air jet is too weak, then themixing is subsequently weak. This air system was designed to limit NOxemissions in Scandinavia and is successful but limited in other ways andexpensive to retrofit. For example, the production of NOx in a recoveryboiler is a function of the combustion temperature. Higher temperaturesform more oxides of Nitrogen than cooler combustion temperatures.Therefore the vertical air system is designed to “stage” the combustionto keep the peak flame temperatures down. While this helps to controlNOx formation it may not reduce total reduced sulfur (TRS), and may notimprove reduction efficiency, heat transfer, or char bed control, andmay delay final combustion until high in the boiler where higher gastemperatures can be a problem as described earlier. The vertical airsystem is expensive to retrofit because more than seven levels of portopenings and ducting must be installed. Where the vertical air system issuccessful is in creating vertical mixing zones in the furnace thatimprove the mixing and combustion to the extent that combustion air islimited (i.e. staged), but not all openings nor levels aresimultaneously in use. While reducing NOx emissions is valuable, theoverall benefit of the vertical air system is limited and theimplementation is expensive. The invention described herein is animproved combustion air system that controls NOx emissions while alsoimproving reduction efficiency, improving heat transfer to the boilerwalls, improving boiler water circulation, improving char bed control,reducing carryover, reducing gas temperatures in the upper furnace, andreducing TRS and CO emissions, and is economical to implement.

SUMMARY OF THE INVENTION

In accordance with the invention, an improved combustion air system isprovided for a recovery boiler in which multiple levels of secondary andtertiary combustion air ports each have an even number of ports, withthe ports on opposing walls interlaced. The air system is adapted forfront/rear wall or sidewall applications. The system features large andsmall-scale horizontal circulation zones superimposed on each other andthe angle of the air jets is adjustable. Interlaced or inboard/outboardspacing may be employed. Port sizes can be adjustable to modify air flowfrom a selected port. The air flow of ports can be the same as others,or may be different.

Accordingly, it is an object of the present invention to provide animproved combustion air system for a recovery boiler.

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of this specification.However, both the organization and method of operation, together withfurther advantages and objects thereof, may best be understood byreference to the following description taken in connection withaccompanying drawings wherein like reference characters refer to likeelements.

BRIEF DESCRIPTION OF THE DRAWINGS

—FIG. 1 is a front and side view diagram of a recover boiler air system;

FIG. 2A is a top view of an interlaced configuration FIG. 2B is a topview of an inboard/outboard configuration;

FIG. 3 is a diagram illustrating small cyclones of air jets;

FIG. 4 is a circulation flow diagram of a larger circulation around therecovery boiler;

FIG. 5 is a view of a front wall of a furnace;

FIG. 6 is a top view illustrating air flow from ports;

FIG. 7 is a side view of a recovery boiler illustrating air flow;

FIG. 8 is a top view illustrating large-scale rotations of the flue gas;

FIGS. 9A and 9B illustrate large-scale rotation superimposed on severalsmall-scale rotations;

FIG. 10 is another recovery boiler side view; and

FIG. 11 illustrates angling the secondary and/or tertiary air jetsdownward.

DETAILED DESCRIPTION

The invention consists of an air system with one or two primary level,typically two secondary levels, and typically two tertiary levels.FIG. 1. Each of the secondary and tertiary levels has an even number ofports, for example, three ports on the front wall and three ports on therear wall. FIG. 2A. The ports on the opposing walls are arranged in aninterlaced fashion such that (for example) the front wall ports blow inbetween the rear wall ports and vice versa. This arrangement requiresthat the ports on one wall be offset from the ports on the oppositewall. For example the front wall ports may be pushed toward the leftsidewall and the rear wall ports may be pushed toward the right sidewall. Port spacing may be arranged as follows (assuming ports are onfront and rear walls): If the width of the boiler is W, and there are Nnumber of ports at a given level (for example three front+three rear=6total), then the spacing from the sidewall to the first port is W/(N+1),and the spacing between ports is 2W/(N+1). These are example spacings,not requirements, and other spacings may be desirably employed.

FIG. 2B illustrates an alternative configuration, calledinboard/outboard, wherein on one sidewall there are two (in theillustrated example) ports spaced laterally farther apart, whereas onthe opposing wall, there are two ports which are spaced relatively near,wherein the two near ports are positioned within the interior boundarydefined by the distance between the two far apart ports of the oppositewall.

The centerline of the lowest secondary level is located about 1 meterabove the centerline of the lowest primary port level. If the floor ofthe boiler is sloped, and the primary port elevations follow the slopeof the floor, the lowest secondary level is about 1 meter above thelowest primary ports at the high end of the floor. The upper secondarylevel is located about 1 meter above the lower secondary level. Thelower tertiary level is located from two to four meters above the liquorspray and the upper tertiary located from one to three meters above thelower tertiary level. These dimensions are referenced to the portcenterlines. At the secondary level, the ports are arrangedsubstantially directly above each other. At the tertiary level the portsare also arranged substantially directly above each other and directlyabove the secondary. This arrangement creates reinforced mixing zones asthe fuel is burned and the gasses are rising from the secondary to thetertiary level. For example a small cyclone of gasses is created betweeneach pair of interlaced air jets and this pattern is reinforced as longas the ports are directly above each other. FIG. 3. Because the portsare offset with one wall to one side and the opposite wall to the otherside, a larger circulation is created that flows slowly around theperimeter of the boiler. FIG. 4. With the tertiary ports above thesecondary ports the small cyclones and the larger rotation arereinforced. The smaller and larger rotational patterns have the effectof increasing the flow path and residence time of the combustible gassesgiving them more time to burn out before they must exit the furnace. Thepreferred embodiment is to have the tertiary ports arrangedsubstantially directly above the secondary ports to maintain the smalland large rotations. In some cases it may be preferred to reverse thebias direction between the secondary and tertiary levels, which wouldtend to reverse the small rotations and cancel out the large rotations.For example, looking at the front wall of the furnace, if the left handsecondary ports are W/(N+1) from the left wall, the right hand tertiaryports may be W/(N+1) from the right wall. FIG. 5.

Most modern secondary and tertiary air systems utilize an odd number ofports (total per level) interlaced on the front and rear walls. FIG. 6.This arrangement balances the gas flows side to side across the width ofthe boiler to encourage even flows into the upper furnace. The imbalancefront to back is mitigated by the influence of the nose arch in the rearwall of the boiler. The nose arch is a portion of the rear wall thatbends out into the furnace cavity and directs the gas flow away from therear wall and channels it across the superheater. FIG. 7. The nose archalso shields the convection surfaces from radiant heat from thefireball. If there are three ports at each level on the front wall andtwo on the rear wall, typically 60% of the secondary and tertiary aircomes from the front wall, crosses the boiler and rises toward the rearof the furnace where it is intercepted by the nose arch. Asecondary/tertiary air system with an even number of ports at each level(e.g. three interlaced with three) has inherently balanced flows frontto back and side to side with the further advantage of creating a slowmacro rotation due to the offset. In the past, large-scale rotations ofthe flue gas (typically from tangential firing, FIG. 8.) have beenconsidered detrimental because of the tendency to form high-speedvertical cores that exacerbate the carryover and boiler plugging. Thepresent embodiment, however, can be adjusted to, if desired, utilize alarge-scale rotation superimposed on several small-scale rotations suchthat the overall flow path and residence time of the gasses is increasedwhile maintaining low vertical velocities. FIG. 9. The result is thatmore of the furnace volume is used for combustion and heat transfer andfor transporting the gasses upward. This reduces the average verticalgas velocity reducing carryover, upper furnace temperatures, andplugging, and improves heat transfer to the boiler walls.

The previously mentioned tangential firing systems have the additionalinherent disadvantage of the rising gasses skewing off to one side ofthe boiler. Therefore the high-speed column of gasses becomesconcentrated to one side which imbalances the boiler loading andpreferentially plugs one side of the furnace and under utilizes thesuperheater. Many conventional air systems with opposed secondary andtertiary ports (non-interlaced) have the same problem. This illustratesthe disadvantage of side-to-side imbalance. Air systems with an oddnumber of ports at the secondary and tertiary levels do not lendthemselves to sidewall applications because they will create animbalance in the gas flows side-to-side. For example, if three ports onthe right sidewall are interlaced with two ports on the left sidewall,60% of the gasses will rise on the left side of the boiler causing animbalance. By contrast, the present invention has an even number ofports at each level therefore it can be applied as functionally to thesidewalls as to the front and rear walls. The ability to implementsidewall secondary and tertiary air systems can become important forrectangular boilers that are deeper than they are wide. Also, it islikely more important when the smelt spouts are located on the sidewalls. In these cases a sidewall air system better utilizes the planarea and volume of the furnace because the combustion air ports can bespaced further apart and do not have to carry as far across the boiler.It is also beneficial when implementing a front wall/rear wall system isprohibitively expensive due to physical constraints around the boiler.Also, it may be desired in some cases to locate the ports on the samewalls as the spouts.

In addition to the combustion air ports there are several other openingsin the boiler walls including smelt spout openings, camera openings,starting burners, liquor gun openings, load burners, particulators, etc.FIG. 10. All of these are sources of additional air entering the boiler.The smelt spout openings and liquor gun openings are typically open tothe atmosphere therefore the amount of air that enters is minimal. Theburner openings by contrast are typically large with four to six or morestaring burners and zero to five or more load burners. The startingburners are often a large source of air entering the boiler because thecombustion is intense at this level and it is necessary to cool theburners when not in use. This cooling air may account for 15% or more ofthe total combustion air. In typical combustion air systems, this air isat low velocity and not directed in a manner that improves thecombustion in the furnace. On many boilers, the air entering through thestarting burners is detrimental because it impedes the performance ofthe secondary air system and adds to the upward gas velocity withoutcontributing to mixing the fuel and air. The present invention includesa means to reduce or eliminate burner-cooling air by closing thestarting burner ports with a refractory lined damper, the refractorybeing necessary to protect the damper from the intense heat ofcombustion. Most recovery boilers, have the starting burners located onthe sidewalls (although it is not required that they be on thesidewalls). As an alternative to the damper system, if the combustionair ports are located on the sidewalls, they can be combined with theburner ports so that the combustion air also cools the burners. Sidewallsecondary systems are common place, some combined with the burner ports,however they all suffer from either too many ports, improper arrangementor ports that are too small. The present invention facilitates sidewallsecondary and tertiary combustion air systems thereby solving theseproblems.

Burner ports tend to be bigger than air ports, so in the case of usingthe burner ports as airports also, in accordance with the presentinvention, dampers are employable to adjust the air flow from a modifiedburner port to be equivalent of a typical air port. The damper adjuststhe size of the burner port to allow control and adjustment of the airflow.

Another embodiment of the present invention includes angling thesecondary and/or tertiary air jets downward. FIG. 11. At the secondarylevel this directs the lower air jets downward to the char bed toimprove char bed control. At the tertiary level, the lower tertiaryports are frequently placed higher than desirable due to constraintsaround the boiler (e.g. the tertiary operating floor). In these casesthere is a volume of space above the liquor spray but below the tertiarylevel that is underutilized. By angling the tertiary jets downward thisspace can be better used for combustion. Also, the upward pressure ofthe rising gasses on the tertiary air jets tends to bend the air jetsupward. This increases the vertical velocity component of the air jets.Angling the air jets downward tends to overcome this upward pressure.Angling the combustion air jets downward has been common practice at theprimary level and in the past at the tertiary. The present inventionhowever, includes the refinement of the system whereby the lower of thesecondary or tertiary jets is angled downward while the upper-level iskept horizontal. This arrangement utilizes more of the lower furnacevolume for combustion. Also, the lower jet protects the upper jet frombeing deflected upward more than necessary.

It may be desirable to change the angle of the air jets depending onload rate, char bed conditions, black liquor changes, etc. The presentinvention includes the ability to adjust the air jet angle in thevertical direction using a device similar to the Directional AutoportSystem described in U.S. Pat. No. 6,497,230 (Higgins et al.), copyenclosed. In this manner the invention departs from common engineeringpractice.

It is also desirable to balance the airflows from the secondary andtertiary levels such that they contribute equally to the mixing andcombustion, and so that the rotational patterns are controlledthroughout the furnace. Because the different levels may operate atdifferent mass flows, temperatures, and velocities, it is not adequateto balance the system based directly on these parameters. Rather thesystem is balanced based on the kinetic energy of the air jets. Kineticenergy is defined as the mass flow times the velocity squared.(E_(k)=mv²). In this manner, regardless of the differing mass flows andtemperatures, each air jet can be adjusted to contribute equally to thecombustion air system. This is important to maintain desired rotationalpatterns in the furnace and achieve the results predicted by the CFDmodels. The mass flow and velocity are typically controlled by adjustingthe port opening using devices similar to U.S. Pat. Nos. 5,001,992 and5,307,745 (Higgins et al), copies enclosed, and by adjusting the staticair pressure. For example, which high mass flow and low velocity ispresent, the balance is made based on momentum, but if low mass and highvelocity, then balancing is done based on kinetic energy.

In the system described, the amount of flow from the ports does not needto be the same. The flow can be adjusted on individual ports to achievedesirable results. For example, by reducing the outermost air flow, theuse of more of the volume of the boiler is accomplished, with reductionor elimination of the large macro flow, while maintaining the smallscale flows.

The system can employ interlaced or inboard/outboard port spacing. Theport sizes can be adjustable. The port sizes do not need to be uniform,although they can be. The systems can inject non-compressible gases atthe secondary or tertiary levels. The majority of the examples hereinillustrate 3 b 3 interlaced systems (FIG. 2A, for example), but othernumbers, such as 2 by 2 interlaced systems, are employable. Systems withside to side symmetry but not front to rear symmetry (e.g. 3 evenlyspace ports on one wall and 2 spaced ports on another wall) can beemployed. Systems with front to rear symmetry but not side to sidesymmetry are also usable, for example, where there are 3 ports on thefront and rear walls, but the spacing between ports on one or both wallsare not uniform. Systems employing the above noted concepts can run atlower combustion air temperatures, which can be desirable.

While plural embodiment of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A combustion air system for a recovery boiler consisting of one ormore primary air levels, two or more secondary air levels, and two ormore tertiary air levels incorporating an two or more number ofcombustion air ports at each of the secondary and tertiary air levelswith half of the ports at each level on a first wall of the boiler, andhalf on the opposing wall, with all of the ports on the first walleither interlaced or in an inboard/outboard configuration with the portson the opposing wall.
 2. A combustion air system according to claim 1 inwhich the first wall is the front wall of the boiler and the opposingwall is the rear wall of the boiler.
 3. A combustion air systemaccording to claim 1 in which the first wall is a sidewall of the boilerand the opposing wall is the other sidewall of the boiler.
 4. Acombustion air system according to claim 1 in which, at any of thesecondary or tertiary levels, the left-most port on the first wall, whenviewed from outside the boiler, is W/(N+1)+/−2 tube spaces from thenearest adjacent wall, where W is the width of the first wall and N isthe total number of ports at that level, and the spacing between theports on the first wall is 2W/(N+1)+/−2 tube spaces.
 5. A combustion airsystem according to claim 4 in which the left-most port on the opposingwall, when viewed from outside the boiler; is W/(N+1)+/−2 tube spacesfrom the nearest adjacent wall and the spacing between the ports on theopposing wall is 2W/(N+1)+/−2 tube spaces.
 6. A combustion air systemaccording to claim 1 in which, at any of the secondary or tertiarylevels, the right-most port on the first wall, when viewed from outsidethe boiler, is W/(N+1)+/−2 tube spaces from the nearest adjacent wall,where W is the width of the first wall and N is the total number ofports at that level, and the spacing between the ports on the first wallis 2W/(N+1)+/−2 tube spaces.
 7. A combustion air system according toclaim 6 in which the right-most port on the opposing wall, when viewedfrom outside the boiler, is W/(N+1)+/−2 tube spaces from the nearestadjacent wall and the spacing between the ports on the opposing wall is2W/(N+1)+/−2 tube spaces.
 8. A combustion air system according to claim1 in which the centerline of the lowest secondary level is 0.5 to 3meters above the centerline of the highest primary port in the lowestprimary level.
 9. A combustion air system according to claim 1 in whichthe centerline of the second lowest secondary level is 0.5 to 3 metersabove the centerline of the lowest secondary level.
 10. A combustion airsystem according to claim 1 in which the centerline of the third lowestsecondary level is 0.5 to 3 meters above the centerline of the secondlowest secondary level but not above the centerline of the liquor gunopenings.
 11. A combustion air system according to claim 1 in which thecenterline of the lowest tertiary level is 0 to 6 meters above thecenterline of the liquor gun openings.
 12. A combustion air systemaccording to claim 1 in which the centerline of second lowest tertiarylevel is 0.5 to 6 meters above the centerline of the lowest tertiarylevel.
 13. A combustion air system according to claim 1 in which thecenterline of third lowest tertiary level is 0.5 to 6 meters above thecenterline of the second lowest tertiary level.
 14. A combustion airsystem according to claim 1 in which one or more secondary or tertiaryports are used to re-circulate boiler flue gas.
 15. A combustion airsystem according to claim 1 in which one or more of the secondary ortertiary ports is used to inject dilute non-condensable gasses into theboiler for incineration.
 16. A combustion air system according to claim1 in which one or more of the secondary or tertiary ports is used toinject concentrated non-condensable gasses into the boiler forincineration.
 17. A combustion air system for a recovery boilerresulting in a large spiral circulation pattern of gasses around theinside perimeter of the boiler superimposed on a series of smallerspiral gas circulation patterns contained inside the large pattern. 18.A combustion air system according to claim 1 in which the uppersecondary ports are located directly above the lower secondary ports+/−2 tube spaces, and 0.5 the upper tertiary ports are located directlyabove the lower tertiary ports +/−2 tube spaces.
 19. A combustion airsystem according to claim 18 in which all of the tertiary ports on afirst wall are.
 20. located directly above the secondary ports on saidfirst wall +/−2 tube spaces, and all of the ports on said first wall areinterlaced with secondary and tertiary ports similarly located on theopposing wall.
 21. A combustion air system according to claim 18 inwhich the tertiary ports on a first wall of the boiler are locatedhorizontally mid-way between the secondary ports on said first wall +/−2tube spaces, and such that the tertiary ports on the opposing wall arelocated horizontally mid-way between the secondary ports on the opposingwall, and such that the tertiary ports on the first wall are interlacedwith the tertiary ports on the opposing wall, and the secondary ports onthe first wall are interlaced with the 25 secondary ports on theopposing wall.
 22. A combustion air system according to claim 18 inwhich all of the tertiary ports on a first wall are located directlyabove the secondary ports on said first wall +/−2 tube spaces, and allof the ports on said first wall are inboard/outboard with secondary andtertiary ports similarly located on the opposing wall.
 23. A combustionair system according to claim 18 in which the tertiary ports on a firstwall of the boiler are located horizontally mid-way between thesecondary ports on said first wall +/−2 tube spaces, and such that thetertiary ports on the opposing wall are located horizontally mid-waybetween the secondary ports on the opposing wall, and such that thetertiary ports on the first wall are inboard/outboard with the tertiaryports on the opposing wall, and the secondary ports on the first wallare interlaced with the secondary ports on the opposing wall.
 24. Acombustion air system according to claim 1 wherein the configuration isinboard/outboard, and the flow for at least one outboard port isdifferent than flow for an inboard port.
 24. A combustion air systemaccording to claim 1 in which the secondary combustion air mass flowsare balanced front to back and side-to-side +/−20%.
 25. A combustion airsystem according to claim 1 in which the secondary combustion air massflows are balanced wherein the outermost ports and the innermost 2portshave a flow difference relationship of approximately 50%.
 26. Acombustion air system according to claim 1 in which the lower boiler isrectangular in plan form with the front and rear walls longer than thesidewalls and the secondary and tertiary ports located on the front andrear walls.
 27. A combustion air system according to claim 1 in whichthe lower boiler is rectangular in plan form with the sidewalls wallslonger than the front and rear walls and the secondary and tertiaryports located on the sidewalls.
 28. A combustion air system according toclaim 1 in which the lower boiler is rectangular in plan form with thesidewalls walls shorter than the front and rear walls and the secondaryand tertiary ports located on the sidewalls.
 29. A combustion air systemaccording to claim 1 in which the starting burner port openings arefitted with adjustable dampers covering the port openings at the furnacewall.
 30. A combustion air system according to claim 28 in which thedampers are refractory lined.
 31. A combustion air system according toclaim 1 in which one or more of the secondary ports incorporates anauxiliary fuel burner.
 32. A combustion air system according to claim 1in which one or more of the tertiary ports incorporates an auxiliaryfuel burner.
 33. A combustion air system according to claim 1 in whichone or more of the secondary or tertiary air jets are angled from thehorizontal from 0° to as much as 30° down angle.
 34. A combustion airsystem according to claim 32 in which the angle of the air jets isadjustable from 0° to 30° down angle while the boiler is on line andfrom outside the boiler.
 35. A combustion air system for a recoveryboiler in which the secondary and tertiary combustion air flows arecontrolled on a kinetic energy basis.
 36. A combustion air systemaccording to claim 34 in which the secondary air jets and the tertiaryair jet have substantially the same kinetic energy.
 37. A combustion airsystem according to claim 34 in which the secondary and tertiary airjets are adjusted to contribute substantially equally to the fuel airmixing in the boiler.
 38. A combustion air system according to claim 34in which the kinetic energy of any individual or group of secondary ortertiary air jets is held substantially constant as the mass flow and/orwindbox pressure is adjusted.
 39. A combustion air system according toclaim 1 wherein at least one of said air ports is also operative asstarting or load burner.
 40. A combustion air system according to claim38 wherein said at least one air port that is also operative as startingor load burner further comprises a damper for adjusting the air flowthereof.